prog_pot = gp.PlummerPotential(m=prog_mass, b=2*u.pc, units=galactic)
gen2 = ms.MockStreamGenerator(df, H, progenitor_potential=prog_pot)
stream2, prog = gen2.run(prog_w0, prog_mass,
                         dt=1 * u.Myr, n_steps=1000)
stream2.plot(['x', 'y'], marker='o', s=4,
             color='k', alpha=0.1, linewidth=0)